1. Field of the Invention
The present invention relates to co-channel interference filtering for digital television signals, and more particularly to methods and apparatus for detecting the presence of co-channel interference in order to selectively apply a co-channel interference filter.
2. Description of the Related Art
Conventional broadcast television signals are strictly analog in nature. These conventional signals generally conform to one of three broadcast formats in wide adoption: the NTSC (National Television Standards Committee) format adopted in the United States and a few other countries, and the PAL (Phase Alternation by Line) and SECAM (Systeme Electronique Couleur Avec Memoire) formats adopted in most other countries.
High-Definition Television (HDTV), or more generally Digital Television (DTV), formats abandon the conventional analog television signal format in favor of a digitally coded signal. Due to the high redundancy found in most video signals, it is possible to digitally compress a video sequence in a manner that will be visually imperceptible (or mostly so) once uncompressed. Such DTV signals can therefore transmit much more detail than is possible with an equivalent analog signal of the same bandwidth. With the current HDTV format being implemented in the United States, HDTV bandwidth has been set to occupy roughly the same bandwidth as an analog NTSC broadcast, with channels assigned from the same channel space as NTSC channels.
Although the long-term plan is to phase out NTSC channels, the vast majority of television users do not yet own HDTV receivers and a complete switchover does not appear imminent. In the interim, television stations that broadcast an HDTV signal may have viewers that receive both the desired HDTV signal and a relatively strong but undesired NTSC signal on the same channel. In this circumstance, the NTSC and HDTV signals interfere with each other, producing what is known as “co-channel” interference.
Referring to FIG. 1, the frequency spectrum 100 of an NTSC signal is depicted. Envelope 110 represents the DTV information transmitted within the NTSC signal channel space when a DTV signal occupies the same channel space as an NTSC signal. Video carrier V, located 1.25 MHz from the lower edge of the allotted frequency spectrum, is used to demodulate the luminance component of the original NTSC signal. Color subcarrier C, located 3.58 MHz above video carrier V, is used to demodulate the NTSC quadrature chrominance signals in a color television receiver. Audio carrier A, located 4.5 MHz above the video carrier, is used to demodulate the NTSC frequency-modulated (FM) audio signal transmitted in a relatively small frequency band centered about carrier A.
When an HDTV signal occupies the same channel space as an NTSC signal, the NTSC signal can produce strong interference. It is therefore desirable to pre-filter the received HDTV signal with an NTSC rejection filter that can remove predictable components of the NTSC signal, i.e, the video, color, and audio carriers. Typically, a comb filter is used as the NTSC rejection filter. As shown in FIG. 1, the comb filter 120 has nulls spaced 57 fH Hz apart, where fH is the horizontal scan frequency of the analog video signal (15.734 kHz for NTSC video). One comb filter null aligns approximately with the video carrier V, another comb filter null aligns approximately with the color subcarrier C, and a third comb filter null aligns approximately with the audio carrier A.
From FIG. 1, it can be appreciated that the comb filter contains other nulls within the HDTV channel space that in all likelihood will not improve co-channel interference. In fact, the NTSC rejection filter degrades the signal-to-noise ratio (SNR) of the HDTV signal by approximately 3 dB when no NTSC signal is present. Therefore, when the NTSC co-channel interference is less than 3 dB, it is preferable to not use the filter at all. Accordingly, it is recommended that the NTSC rejection filter be switched out of the HDTV signal path when no NTSC signal is present.
Several methods have been used to determine whether an NTSC signal is present in an HDTV channel. U.S. Pat. No. 6,201,576 teaches calculating two noise power estimates on the received signal, one with the NTSC rejection filter in place and one without. An NTSC signal is detected when the difference between the two noise power estimates exceeds a threshold. When the NTSC signal is detected, the NTSC rejection filter is switched into the HDTV processing stream.
U.S. Pat. No. 6,421,077 teaches a slightly different method for determining whether NTSC co-channel interference is present. The error/detecting correcting circuitry in the HDTV receiver outputs an error rate for the received digital signal. This error rate is compared for two conditions—one with an NTSC rejection filter in place, and one without. The configuration that results in the lowest error rate is chosen as the HDTV signal processing path.